Rare sugars are defined as monosaccharides and their derivatives that are rare in nature. These rare sugars have many potential uses in medicine and food industry. Their derivatives can be used as antiviral and anticancer drugs. Most rare sugars taste sweet, but they are not metabolized by humans to the same level of more common sugars. Thus they can be used as low calorie sweeteners. Some rare sugars have the ability to suppress blood glucose and insulin levels and could thus be used as diabetes drugs. Also other medicinal and nutriceutical properties have been found. Rare sugars are generally expensive, as they cannot be isolated from natural sources in significant amounts. They are often produced by chemical means, but due to several steps and poor yields, the prices have stayed high. Biotechnological production is often superior to chemical methods once suitable enzymes acting on rare sugars are found. In this thesis the microbial production of two rare sugars – L-xylulose and L-xylose – and one sugar alcohol, xylitol, was studied. As a meso-sugar, xylitol is an intermediate between D- and L-sugars. It has several interesting characteristics, such as the ability to prevent dental caries and ear infections. In this thesis xylitol was produced from D-xylose by recombinant Lactococcus lactis harboring the xylose reductase gene from Pichia stipitis. The strain was able to produce xylitol from xylose in a quantitative yield, but it was not able to convert all the xylose initially present to xylitol. The productivity of the strain was competitive with previous reports using natural or recombinant strains. Next, the xylitol-4-dehydrogenase from Pantoea ananatis was produced recombinantly in Escherichia coli and characterized. The resting recombinant cells were used to produce L-xylulose from xylitol in quantitative yields. The volumetric productivity of this strain was one of the best reported to date. Last, the L-fucose isomerase gene from E. coli was overexpressed in a recombinant E. coli strain. The gene product was characterized and it was noticed to convert L-xylulose to L-xylose with the reaction balance strongly on the side of xylose. Contrary to earlier reports on the conversion between L-xylulose and L-xylose, practically no L-lyxose was formed with this enzyme. Resting cells of the recombinant strain were also able to convert L-xylulose to L-xylose. In conclusion, starting from a readily available sugar, D-xylose, xylitol and two rare sugars, L-xylulose and L-xylose were produced using resting cells of recombinant bacteria. Additionally, the enzymes performing the reaction between xylitol and L-xylulose, as well as between L-xylulose and L-xylose were characterized.
|Translated title of the contribution||Ksylitolin, L-ksyluloosin ja L-ksyloosin mikrobiologinen tuotto|
|Publication status||Published - 2013|
|MoE publication type||G5 Doctoral dissertation (article)|
- rare sugars
- metabolic engineering